Add option to set from stdin using railway variables

[jzeuzs]

Resolves #635.

[mikewesthad]

Couple small things but otherwise LGTM. I think being able to do:

echo "TEST=123" | cargo run variables

W/o the flag is a little cleaner, but I think what you have with an explicit flag is probably best here for backwards compat

[jzeuzs]

Couple small things but otherwise LGTM. I think being able to do:

echo "TEST=123" | cargo run variables

W/o the flag is a little cleaner, but I think what you have with an explicit flag is probably best here for backwards compat

I think having an explicit flag would be better.

[ndneighbor]

Going to work on Milo's, and then yours after this one, but keeping an eye on this.